A block diagram of a voltage regulator that supplies a load L through a cable C is depicted in FIG. 1. A control system keeps the voltage generated by the converter at a constant value when changes of the input voltage Vin and/or the load L occur.
Optionally, a second control system may be present to regulate the current delivered by the converter.
The two control systems are mutually exclusive: if the current demanded by the load is lower than the current regulation setpoint, the voltage control system will regulate the output voltage and the current control system will be inoperative; contrarily, the current control system will take over and the voltage loop will be inoperative. Voltage control and, when present, current control use a closed-loop negative feedback: the voltage generated by the converter and current through the load, respectively VOUT and IOUT, are fed back to the error amplifiers EAV and EAC and they are compared with their references VREF and IREF, respectively.
The input signals VCV, VCC to the controller come from the error amplifiers that sense the difference between reference values (VREF and IREF) and the feedback signals (VOUT and IOUT). Depending on the input signals, the controller generates a PWM signal that drives power switches. Through a transformer, an output rectifier and a filter, energy is transferred from the supply voltage source VIN to the load L. The diagram shown in FIG. 1 is quite general and may have several possible alternative embodiments.
Typically, energy is transferred to the load through a cable C. The voltage control loop keeps the voltage Vout regulated but, depending on the output current, the voltage on the load, VLOAD, will be affected by a voltage drop along the cable, out of the control loop. Thus if a zero load regulation is to be achieved, it may be necessary to compensate the drop along the cable in some way.
A simple known way of meeting this potential need is illustrated in FIG. 2 and consists in using an additional sensing wire to sense the voltage VLOAD. In this way a zero load regulation may be achieved, but an additional wire is needed. A three-wire cable is not as common as a two-wire one and may be more expensive.
Another solution, that avoids the need of additional wires, is to adjust the voltage loop reference (VREF) by an amount proportional to the average output current, the value of which can be sensed directly even with a remote load. Cable drop compensation (briefly CDC) can be performed if the value of the cable resistance Rcable is known. This solution is depicted in FIG. 3.
The transfer function of the CDC block is:V′REF=VREF+kCDC·IOUT,where kCDC is the cable drop compensation gain and VREF is the adjusted reference.
In the circuit of FIG. 1, during voltage regulation, it is:VOUT=kCV·VREF andVLOAD=VOUT−Rcable·IOUT,where kCV is the voltage loop gain, VOUT is the regulated voltage and VLOAD is the real voltage on the load.
With reference to the diagram of the FIG. 3 the output voltage is:V′OUT=kCV·V′REF=V′REF=kCV·(VREF+kCDC·IOUT)=VOUT+kCV·kCDC·IOUT.
As the resistance Rcable is known by the application, the kCDC value is chosen in order to satisfy the condition VLOAD=VOUT, hence:
            k      CV        ·          k      CDC        =                    R        cable            ⇒              k        CDC              =                            R          cable                          k          CV                    .      
Typically, the output current is sensed directly.
A common way of sensing the output current and adjusting the voltage reference proportionally in a non-isolated step-down switching converter is illustrated in FIG. 4 (from the STMicroelectronics AN1061 applications note, all versions of which are incorporated by reference). In particular, by connecting the resistor RK as shown in FIG. 4, it is possible to adjust the voltage reference value by shifting the ground voltage of the IC by an amount proportional to the current ILOAD.
A similar technique applied to an isolated flyback switching converter is shown in FIG. 5 (from the STMicroelectronics TSM1052 datasheet, all versions of which are incorporated by reference). Only the secondary side is shown; VOUT and IOUT are sensed and compared against their respective references; the error signal (of the loop in control) is transferred to the primary side via an optocoupler, where it is properly handled.
A typical isolated flyback configuration using the optocoupler to transfer the output information from secondary side to the primary one is shown in FIG. 6 (from the STMicroelectronics Viper53 datasheet, all versions of which are incorporated by reference).
There is a special class of low-cost isolated converters, in which output voltage regulation is quite loosely specified and use a simpler approach, according to which there is no sensing element or any reference on the secondary side and, therefore, no specific means for crossing the isolation barrier to transfer the error signal to the primary side, as depicted in FIG. 7 (from the STMicroelectronics Viper53 datasheet, all versions of which are incorporated by reference). In these systems, the voltage drop along the output cable adds to their inherently poor load regulation and can make unacceptable the use of such low-cost systems. In this case, a cable drop compensation circuit would make the difference. However, there is no known technique to compensate the cable resistance for this type of switching converter.